DESCRIPTION: (Adapted from Abstract) During the follicular phase the capacity of preovulatory follicles to produce androgens and estradiol increases greatly, whereas progesterone secretion remains low. In vivo the LH/FSH surge triggers several dramatic and important changes, including the follicular/ luteal phase shift in steroidogenesis from androgen/estradiol to progesterone/ oxytocin. Little is known about the mechanisms of this shift. Cattle offer unique advantages as a model for studying the follicular/luteal shift because preovulatory follicles can be obtained at defined developmental stages and provide large numbers of granulosa and theca cells and behavioral estrus serves as a marker for the LH/FSH surge. We have found that a narrow range of very low doses of gonadotropins (1-4 ng/ml) maintains follicular phase-type hormone secretion in cultured follicle cells, higher doses accelerate luteinization in vitro. We propose to address the question of how low (basal) vs. high (surge) concentrations of the same hormones (LH and FSH) elicit dramatically different responses in theca and granulosa cells. Specific Aim 1 will test the hypothesis that, during the follicular phase, follicle cells acquire the competence to luteinize such that in vitro they can retain functions of follicle cells or luteinize, depending on the hormonal milieu provided. This will be approached by : 1) determining if low concentrations of LH or FSH maintain follicular phase-type functions and high concentrations promote luteal functions in bovine theca and granulosa cells via differential use of PKA vs. PKC pathways (Specific Aims 2,3), 2) determining the role of differential expression of mRNA for steroidogenic enzymes, oxytocin, and StAR (steroidogenic acute regulatory protein) (Specific Aim 4). The potential role of an endogenous PKC inhibitor (PKCI) in regulation of luteinization of follicle cells will also be examined (Specific Aim 5).